42 INVERTEBRATA OF MASSACHUSETTS. 



the shell varies according to its exposure to the chafing of the sea. 

 Some specimens, obtained in the still, sandy harbour of Provincetown, 

 are very white, and nearly as thin as paper. 



In the young shell the valves are quite unequal, and the tooth is 

 produced towards the longer side, so as to be somewhat triangular. I 

 have compared shells in this state, a third of an inch in length, with 

 specimens of SPHENIA Swainsbni, TURT., and can find no differences 

 in the hinge, and none in the shell, unless that perhaps the latter may 

 be a little thinner, and proportionally longer than the former. 



MYA TRUNCA'TA. 



Shell oblong-oval-, inequilateral, rounded anteriorly^ truncated 

 posteriorly ; icidely gaping ; tooth broader than long. 



State Coll., No. 233. Soc. Cab., No. 2319. 



Mya trunckta, LIN. ; Syst. Nat., 1112. Gmel. No. 1. FABR.; Fauna GrenZ.,404. 

 PENNANT; Prit. Zoo/., iv. pi. 41, f. 14. CHEMN. Conch., vi. 8, tab. 1, f. 1, 2. 

 MONTAGU; Test. Brit.^ 32. MATON and RACKETT ; Lin. Trans., viii. 35. WOOD; 

 Gen. Conch., 90, t. 17, f. 1,2. Index, pi. 2, f. 1. DILLWYN ; CataL, i. 42. DES- 

 HAYES ; Encyc. Mith., Vers, ii. 501, pi. 229, f. 2 to 6. DONOVAN ; Brit. Shells, 

 pi. 92. LAM., An. sans Vert. vi. 73. TURTON ; Conch. Diet., 97. Brit. Biv., 31. 

 LISTER ; Conch., t. 428, f. 269. GUALT. ; Test., t. 91, f. D. BROOKE ; Jntrod., 

 pi. 1, f. 10. 



Shell oblong, inequilateral, longest and rounded before, nar- 

 rower and abruptly cut off, generally obliquely, behind ; the valves 

 are strong, deeply concave and often unequal, but sometimes the 

 right valve, and sometimes the left, is most prolonged ; surface 

 irregularly ridged along the lines of growth ; color dingy white, 

 covered with a thick, tough, yellowish, wrinkled epidermis, 

 which folds over the edges of the shell, and is greatly prolonged 

 postefiorly, forming a tube six or eight inches long, supplying in 

 some measure the apparent loss of protection to the animal by the 

 truncation of the shell. The truncated edges are a little flaring, 

 and the shell is left wide open behind ; beaks moderately promi- 

 nent, turning slightly forwards ; teeth broader than long, projecting 

 a little inwards ; inner face smooth, and nearly flat ; outer face 

 similar to that of M. arenaria, but the oblique rib merely forms a 

 thickened lobe at the edge, and does not project into a tooth-like 

 process ; on the opposite valve is an excavation in the beak for 



